Successful orthopaedic surgery, for example, the replacement of hip and knee joints with endoprostheses, the correction of knee deformities such as genu valgum (knock knee) and genu varum (bowleg) by osteotomy requires precise knowledge of the position of the articular point of the joint being operated on. The articular point represents an imaginary joint center about which the bones connected at the joint rotate. More precision in the knowledge of the articular point of a joint results in longer lasting replacement joints and more effective correction of deformities.
To improve orthopaedic surgery, navigation systems have been developed, such as disclosed in U.S. Pat. No. 6,385,475 to Cinquin et al and hereby incorporated by reference. Such systems use markers attached to bones on opposite sides of the joint connecting the bones. The markers are observable by a stereoscopic camera system connected to a data processing system such as a computer that can record the positions of the markers in space and, using software, calculate the kinematic motion of the bones, as well as other mathematical parameters and relationships. The markers attached to the bones establish a coordinate reference system relative to each bone. Additional camera observable markers are freely moveable and may be used to palpate (touch) specific landmarks on the bones in order to ascertain the position of the landmarks in the coordinate reference systems of the bones. The positions of such landmarks are used by the data processing system software, along with the relative motion of the bones connected at the joint of interest, to calculate the geometric and kinematic relationships needed to guide the orthopaedic surgery. Included among these parameters are articular points or joint centers.
Present methods for determining the position of articular points, such as the knee center, require that the knee joint be surgically opened to provide access to the anatomical center of the knee (a landmark point on the femur) so that this point may be palpated by a movable marker to establish its precise location in space relative to the femur coordinate reference system defined by the marker attached to the femur. Using the location of the anatomical knee center in conjunction with the positions of other landmarks (such as the medial and lateral epicondyles, also determined by palpation), as well as motion of the tibia relative to the femur, the data processing system software can calculate a relatively accurate position of the knee center in the femur and tibia coordinate systems. The position of the knee center is then used to provide further information directing the placement of endoprostheses or guiding the bone cutting in an osteotomy.
Methods involving surgically opening the knee joint to determine the position of the knee center are acceptable when the contemplated operation also requires access to the knee joint, such as during a total knee arthroplasty (knee replacement). However, for less invasive procedures, such as the mere gathering of information for pre-operative diagnostic purposes, or an osteotomy to correct a knee deformity, that do not require the knee be surgically opened, it is not advantageous to open the knee merely to palpate the anatomical center to ascertain the knee center location. Clearly, there is a need for a non-invasive method for determining the position of the knee center, as well as the articular points of other joints.